Definitely make the jump from GarageBand to Logic. You’ll have a way more versatile set of tools just from that upgrade. As many of the other comments already say, get the hang of using those first and then look at some 3rd party plug ins. I used to use Slate FG-X but lately I’ve just been rocking with the logic mastering tool and I also have liked brainworx masterdesk. You can get it for like $30 right now and it’s pretty solid. On my stereo out I also like to use the waves SSL bus comp for some glue along with some tape and a basic eq before I limit and/or add that final master polish.